The structure of the digestive tract and its evolution are affected by
many factors. Such factors include the type of food eaten, the level of
activity and metabolism, and the size of the animal. The mouth and oral
cavity of the shark has evolved according to the type of food the shark
eats. The shark feeds on prey much to large to be sucked in and swallowed
whole, as suction feeders do. The jaws and teeth seize prey, and it is
torn into chunks that can be ingested. The shark tooth has evolved from
a smooth round tooth to a sharp, serrated triangular tooth. Since these
teeth are attached only to the connective tissue of the jaws, sharks have
evolved a continuous growth replacement system for the teeth. The shark
jaw is also adapted to feeding on larger prey. Sharks are said to have
kinetic skulls where considerable movement is possible between the jaw
and other parts of the skull. The jaw joint has shifted rostrally and the
elements of the jaw have become hinged to each other, and to the rest of
the skull, in a way that when the jaw is lowered, the entire mouth is carried
forward toward the prey. The tongue of the shark has evolved from a primary
tongue of lampreys, which has horny teeth to rasp and hold its prey, to
the muscular tongue. The muscular tongue of the shark can be used to manipulate
its prey.

In most fishes the pharynx, is a large chamber due to its involvement in
both digestion and respiration. The esophagus of the lamprey is the dorsal
part of the larval pharynx, and the ventral part of the pharynx forms the
respiratory tube. However, in sharks the esophagus is a connecting segment
between the pharynx and the stomach. Adult lampreys lack stomachs. Presumably,
the stomach evolved as larger vertebrates began to feed on larger animals
that were captured at less frequent intervals. The stomach functions as
a chamber for the storage of food. This large area for storage allows the
shark’s metabolism to slow down, allowing it to not have to eat for long
periods of time.

The primitive
bony fishes have short, nearly straight intestines. Because of their increased
activity and metabolism, sharks have evolved a high-pitched fold called
a spiral valve. The increased surface area of the intestines increases
the food’s transit time through the intestines, which increases the amount
that can be absorbed. The spiral valve allows the food to be completely
absorbed and digested. The more food that can be absorbed, the less a shark
has to eat.

The liver,
an accessory organ to the digestive system, also has a major evolutionary
significance in the shark. The lamprey liver is a single small lobe behind
the heart. The liver of the shark, on the other hand, is very large and
constitutes 25% of its total weight. Since the shark does not have a swim
bladder, it must reduce its density to obtain bouncy. The liver stores
large amounts of oil, which reduces the shark’s density. It is less advantageous
to have a swim bladder because it takes more time for the gasses to fill
and deflate in the swim bladder. Thus, the shark can move up and down in
the water at a faster rate.